The present invention is related to the field of magnetic devices. More in particular, a magnetic data storage system and a sensing system of magnetic characteristics, the systems having a magnetization direction that is irreversible in an external magnetic field, are disclosed. A method of manufacturing, a method of resetting or changing or repairing and a method of operating such systems are also disclosed.
Magnetic devices are known in the art. Spin-valve structures such as Giant Magneto Resistance (GMR) and Spin-tunnel Magneto Resistance (TMR) devices recently have been extensively studied and were subject of a vast number of disclosures. GMR- and TMR-devices comprise as a basic building stack two ferromagnetic layers separated by a separation layer of a non-magnetic material. This structure in the sequel is referred to as the basic GMR- or TMR-stack of the magnetic device, or is referred to as the GMR- or TMR-structure. Such structure has magneto resistance characteristics and shows the GMR- or TMR-effect. The separation layer is a non-ferromagnetic metallic layer for GMR-devices, and is a non-metallic, preferably insulating, layer for TMR-devices. Over the separation layer, there is a magnetic coupling between the two ferromagnetic layers. The insulating layer in the TMR-devices allows for a significant probability for quantum mechanical tunneling of electrons between the two ferromagnetic layers. Of the two ferromagnetic layers, one is a so-called free layer, and one is a so-called pinned or hard layer. The free layer is a layer whose magnetization direction can be changed by applied magnetic fields with a strength lower, preferably substantially lower, than the strength of the field required for changing the magnetization direction of the pinned layer. Thus the pinned layer has a preferred, rather fixed magnetization direction, whereas the magnetization direction of the free layer can be changed quite easily under an external applied field.
The hard layer can consist of a hard magnetic material or of a relatively soft magnetic material pinned by exchanged biasing to an Anti-Ferromagnetic (AF) layer, or it can consist of an Artificial-Anti-Ferromagnet (AAF) consisting of two or more magnetic layers coupled in an anti-parallel direction by an appropriate intermediate non-magnetic coupling layer. The AAF can be biased by an AF layer to make it even more rigid and to define a single-valued magnetization direction of the AAF.
A change of the magnetization of the free layer changes the resistance of the TMR- or GMR-device. This results in the so-called magneto resistance effect or GMR/TMR effect of these devices. The electrical resistance of the TMR- or GMR-device changes in a predictable manner in response to a varying magnetic field, making the devices suitable for use as magnetic-electrical transducers in a sensing system of a magnetic field. The characteristics of these magnetic devices or systems can be exploited in different ways. For example a spin valve read-out element utilizing the GMR-effect can be used for advanced hard disk thin film read heads. Also stand-alone magnetic memory systems (MRAMs) or non-volatile embedded memory systems can be made based on the GMR- or TMR-devices.
A further application is a sensor device or system for magnetic characteristics. Such sensing systems are used for example in anti-lock braking (ABS) systems or other automotive applications.
It is often required in a number of applications to clearly distinguish between the response of the sensor system (resistance changes) due to (varying) magnetic field and the response of the sensing system (resistance changes) due to environmental factors such as temperature variations. One approach in solving this problem consists in connecting a number of GMR- or TMR-devices in a Wheatstone bridge arrangement. If a pair of GMR or TMR devices can be magnetically biased in such a manner as to have opposite responses (in the sense of opposite polarity) to a given magnetic field but not to other environmental factors, then subtractive comparison of the electrical resistances of the two GMR or TMR devices will cause cancellation of any unwanted response to spurious environmental factors, while exposing any response to magnetic field.
Magnetic field sensing system employing a Wheatstone bridge in this manner are known from the prior art. However, among the sensing system thus known, there are various different approaches when it comes to magnetically biasing the magneto-resistive devices.
For example: in Japanese patent application (Kokai) No. 61-711 (A), each of the resistive devices in the Wheatstone bridge is magnetically biased in a given direction using an appropriately poled permanent magnet positioned in the vicinity of that device; on the other hand, in an article in Philips Electronic Components and Materials Technical Publication 268 (1988) entitled xe2x80x9cThe magnetoresistive sensorxe2x80x9d the individual resistive devices are biased using a so-called xe2x80x9cbarber polexe2x80x9d (a term generally known and understood in the art, and thus receiving no further elucidation here).
The use of biasing on the basis of permanent magnets as in case (a) above is highly unsatisfactory: not only is very careful tuning of the strength and position of the permanent magnets required, but the permanent magnets are themselves unacceptably sensitive to temperature variations. In addition, the use of permanent magnets necessarily makes any such biased magnetic field sensor bulky, and sets a limit on the attainable degree of miniaturization. On the other hand, while the biasing method in case (b) may be suitable for resistive devices demonstrating the so-called Anisotropic Magneto-Resistive (AMR) effect, it cannot be employed in conjunction with resistive devices demonstrating the GMR or TMR effect.
In the prior art document J. Daughton, J. Brown, E. Chen, R. Beech, A. Pohm and W. Kude, xe2x80x9cMagnetic field sensors using GMR multilayerxe2x80x9d, IEEE Trans. Magn. 30, 4608 (1994), two (of the four) bridge elements are magnetically shielded, the shields may be used as flux concentrators for the two sensitive device.
Freitas in xe2x80x9cGiant magnetoresistive sensors for rotational speed controlxe2x80x9d, J. Appl. Phys. 85, 5459 (1999) suggests that two (of the four) bridge devices are xe2x80x9cinactivatedxe2x80x9d by depositing them on a roughened part of the substrate.
Another approach devolves on integrating an isolated conductor below or over the sensor elements (consisting of exchange-biased spin valves) to induce a magnetic field that xe2x80x9csetsxe2x80x9d the exchange-biasing direction of the device in opposite directions, while the devices are heated above the blocking temperature of the exchange-biasing material R. Coehoorn and G. F. A van de Walle, xe2x80x9cA magnetic field sensor, an instrument comprising such a sensor and a method of manufacturing such a sensorxe2x80x9d, patent application EP 95913296.0, now granted, and J. K. Spong, V. S. Speriosu, R. E. Fontana Jr., M. M. Dovek and T. L. Hylton, xe2x80x9cGiant and magnetoresistive spin valve bridge sensorxe2x80x9d, IEEE Trans. Magn.32, 366 (1996); M. M. Dovek, R. E. Fontana Jr., V. S. Speriosu and J. K. Spong, xe2x80x9cBridge circuit magnetic field sensor having spin valve magnetoresistive elements formed on common substratexe2x80x9d, U.S. Pat. No. 5,561,368. A comparable method with an integrated conductor has been proposed for elements based on an Artificial Antiferromagnet (AAF) by W. Schelter and H. van den Berg in xe2x80x9cMagnetfeldsensor mit einer Brxc3xcckenschaltung von magnetoresistiven Brxc3xcckenelementenxe2x80x9d, DE 19520206 (01.06.95).
In the patent application WO 9638738-A1 xe2x80x9cMagnetoresistive thin-film elements-uses adjustment current at high temp. to regulate magnetization distribution of bias layer of sensor elements arranged in bridge circuit, and includes cooling bodyxe2x80x9d (Ger) it is suggested that in the factory the magnetizations are set in opposite directions in different branches of the bridges by exposing a wafer with sensor structures to an external magnetic field that is induced by a kind of xe2x80x9cstampxe2x80x9d comprising a pattern of current carrying conductors which is brought in the vicinity of the wafer.
These prior art solutions are rather complicated and require quite some effort in practice. Moreover, the possibilities disclosed in J. Daughton, J. Brown, E. Chen, R. Beech, A. Pohln and W. Kude, xe2x80x9cMagnetic field sensors using GMR multilayerxe2x80x9d, IEEE Trans. Magn. 30, 4608 (1994), and Freitas xe2x80x9cGiant magnetoresistive sensors for rotational speed controlxe2x80x9d, J. Appl. Phys. 85, 5459 (1999) only allow the realization of a half-bridge and therefore loose half of the possible output signal or response. The magnetic fields that can be realized with the options suggested by R. Coehoorn and G. F. A van de Walle, xe2x80x9cA magnetic field sensor, an instrument comprising such a sensor and a method of manufacturing such a sensorxe2x80x9d and J. K. Spong, V. S. Speriosu, R. E. Fontana Jr., M. M. Dovek and T. L. Hylton, xe2x80x9cGiant and magnetoresistive spin valve bridge sensorxe2x80x9d, IEEE Trans. Magn.32, 366 (1996); M. M. Dovek, R. E. Fontana Jr., V. S. Speriosu and J. K. Spong, xe2x80x9cBridge circuit magnetic field sensor having spin valve magnetoresistive elements formed on common substratexe2x80x9d, U.S. Pat. No. 5,561,368 (04.11.94) and W. Schelter and H. van den Berg in xe2x80x9cMagnetfeldsensor mit einer Brxc3xcckenschaltung von magnetoresistiven Brxc3xcckenelementenxe2x80x9d, DE 19520206 are very limited in strength, because the currents have to be relatively small in the (necessarily narrow and thin) conductors. Further, the options disclosed in J. Daughton, J. Brown, E. Chen, R. Beech, A. Pohm and W. Kude, xe2x80x9cMagnetic field sensors using GMR multilayerxe2x80x9d, IEEE Trans. Magn. 30, 4608 (1994), R. Coehoorn and G. F. A van de Walle, xe2x80x9cA magnetic field sensor, an instrument comprising such a sensor and a method of manufacturing such a sensorxe2x80x9d and J. K. Spong, V. S. Speriosu, R. E. Fontana Jr., M. M. Dovek and T. L. Hylton, xe2x80x9cGiant and magnetoresistive spin valve bridge sensorxe2x80x9d, IEEE Trans. Magn.32, 366 (1996); M. M. Dovek, R. E. Fontana Jr., V. S. Speriosu and J. K. Spong, xe2x80x9cBridge circuit magnetic field sensor having spin valve magnetoresistive elements formed on common substratexe2x80x9d, U.S. Pat. No. 5,561,368 (04.11.94) require several extra processing steps (both for patterning and isolation of the conductors or shields), which makes the sensors more expensive and reduces the fabrication yield. If the method suggested in the patent application WO 9638738-A1 xe2x80x9cMagnetization device for magnetoresistive thin-film elements-uses adjustment current at high temp. to regulate magnetization distribution of bias layer of sensor elements arranged in bridge circuit, and includes cooling body is used, the sensor can be destroyed if exposed to magnetic field of the same strength (or larger) as the field used during setting the magnetization directions.
In particular for automotive applications, but also for read heads, the robustness of the sensing system becomes more and more important. This trend makes setting of magnetization directions after deposition of the elements more and more difficult. In sensing systems that have to operate in relatively large magnetic fields, as required in for instance automotive applications, the hard magnetic layer should be as hard as possible. This makes a definition of the hard magnetization direction after deposition less attractive since it puts an upper limit to the xe2x80x9chardnessxe2x80x9d of the magnetic reference layer, otherwise the magnetization direction of this hard layer cannot be set. For example, the sensing systems as disclosed in WO 96/38740 and WO 96/38738 can not be used in magnetic fields stronger than 15 kA/m (18 mT) since this may change the direction of the hard magnetic reference direction. For automotive applications, typically magnetic bias fields of 5-100 mT are used.
In Wheatstone bridge configurations, it is needed that the four devices that make the Wheatstone bridge are identical and therefore preferably are made under a uniform manufacturing condition. This uniform manufacturing condition may be a uniform deposition condition for all devices of the sensing system but at the end of the manufacturing cycle two devices with opposite exchange biasing directions are needed.
It is an aim of the present invention to disclose a sensing system for a magnetic characteristic and a magnetic data storage system that are robust and that have at least one magnetic characteristic that is irreversible in an external magnetic field. It is another aim of the present invention to disclose a sensing system for a magnetic characteristic that can combine different magnetization directions within a limited space such as a single substrate or a single chip and therefore allows for a further miniaturization of the sensing systems. Also a data storage system is disclosed that can combine different magnetization directions within a limited space such as a single substrate or a single chip and therefore allows for a further miniaturization of the sensing systems. It is a further aim of the present invention to disclose a method of manufacturing a sensing system and/or a magnetic data storage system of which the magnetization direction of at least part of at least one of the devices can be set during manufacturing of the system, and for which the processing is simple and requires only a limited number or no extra processing steps. It is yet another aim of the present invention to disclose a method of manufacturing a sensing system of magnetic characteristics and/or magnetic data storage system that can combine different magnetization directions within a limited space such as a single substrate or a single chip and therefore does not pose strict limits on miniaturization of the system.
Several aspects of the invention are summarized herebelow. The different aspects and embodiments of the invention that are explained in this section and throughout the whole specification can be combined as and to the extent the person of skill in the art is able to appreciate. A number of terms that is used in this summary and throughout the specification is explained at the end of this section.
In a first aspect of the present invention a sensing system of a magnetic characteristic is disclosed. Said system includes a set of magnetic devices in a balancing configuration and essentially each of said devices comprises a structure of layers including at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said structure having at least a magneto resistance effect, and wherein the magnetization direction of the first ferromagnetic layer of at least one of said devices is irreversible in an external magnetic field with a value higher than about 35 kA/m . The value of the external magnetic field can also be higher than about 40 or 50 or 60 kA/m. The external magnetic field can also have value in a range of about 35 kA/m to about 2 MA/m or even 200 MA/m. Preferably the first ferromagnetic layer is the pinned or hard ferromagnetic layer.
The ferromagnetic layers of the devices of the sensing system of the invention may be composed of several layers and other intermediate layers may be present in the stack of layers. In an embodiment of the invention, at least one of the devices of the sensing system includes an Artificial AntiFerromagnet (AAF) structure. An AAF is a magnetic multilayer structure that includes alternating ferromagnetic and non-magnetic layers which have an exchange coupling that results in an antiparallel orientation of the ferromagnetic layers in the absence of an external magnetic field. Such result can be achieved through appropriately choosing the materials and layer thicknesses of the AAF multilayer stack. Each of the ferromagnetic layers of the AAF can consist of subsystems of other ferromagnetic materials. The sensing system can also include an exchange-biased AAF magnetic multilayer structure. The exchange-biased AAF can include an exchange biasing layer of IrMn, FeMn, NiMn, PtMn or NiO type material, said exchange biasing layer being adjacent to, and in contact with, the AAF-structure.
In another embodiment of this first aspect of the invention, the sensing system can comprise at least four (or even at least two) magnetic devices being positioned in a two by two grouped, preferably at least pair-like, configuration with a contact area between the groups (pairs) and with the magnetization direction of the first ferromagnetic layer being substantially opposite for the devices of different groups (pairs) and being substantially identical for the devices of the same group (pair). Preferably the first ferromagnetic layer is the pinned or hard ferromagnetic layer.
In yet another embodiment of this first aspect of the invention, the sensing system can have at least four (or even at least two) magnetic devices being positioned in a grouped, preferably at least two by two pair-like configuration, with a first group (pair) of devices with substantially the same magnetization direction of the first ferromagnetic layer of the devices under an angle of about 90 degrees with a second group (pair) of devices, the second group (pair) of devices having the first ferromagnetic layer with substantially the same magnetization direction but under an angle of about 90 degrees with respect to the magnetization direction of the first ferromagnetic layer of the first group (pair) of devices, and with a contact area. Preferably the first ferromagnetic layer is the pinned or hard ferromagnetic layer.
In a second aspect of the present invention, a method of manufacturing a sensing system of a magnetic characteristic is disclosed. The system includes a set of magnetic devices in a balancing configuration and essentially each of said devices comprises a structure of layers including at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said structure having at least a magneto resistance effect. The method comprises the step of heating part of the system including at least one of the devices of said set while applying an external magnetic field over at least part of said system, the part including said at least one device. The part of the system that is heated can partly or completely coincide with the part of the system that is exposed to said external magnetic field. Thus localized heating of the system in an external magnetic field is achieved. Preferably the external magnetic field is homogeneous over said part. The heating can be achieved by applying a current pulse on or through the device. The heating can also be achieved by applying a laser pulse or a pulse from an electron beam or ion beam on or through the device. Preferably at least one of the devices is heated to a temperature in the range of about 50 to about 800xc2x0 C., preferably in the range of about 300 or 400 or about 600xc2x0 C., while said external magnetic field has a value in the range of about 35 kA/m or 40 kA/m or 50 kA/m to 200 MA/m.
Localized heating of only one of the devices of the system in an external magnetic field can be achieved or all of the devices of said set can be heated to the same temperature or at least two of the devices of the set can be heated to a different temperature value. The external magnetic field directions can also be alternating over at least part of said system. By applying this method of the invention, sensing systems with multiple biasing directions of the devices in the system can be achieved. The devices have a thermally and magnetically robust material structure that is suited in for example automotive applications.
The method can be such that after the execution of the steps, the first ferromagnetic layer of said at least one device has a magnetization direction that is correlated to the direction of said external magnetic field, and preferably being irreversible in an external magnetic field higher than about 35 kA/m.
In a third aspect of the present invention, a method of manufacturing a sensing system of a magnetic characteristic is disclosed. The system includes a set of magnetic devices in a balancing configuration and essentially each of the devices comprises a structure of layers including at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said structure having at least a magneto resistance effect. The method comprises the steps of depositing said structure of layers of at least one of said devices on a substrate while applying at least during a time part of the deposition step an external magnetic field over at least part of said substrate. Preferably, the external magnetic field has at least one characteristic that is position dependent over said substrate. Such characteristic can include the magnitude and/or the magnetization direction of said magnetic field.
During deposition, the substrate can be held in a deposition holder, said holder containing magnetic elements for applying said external magnetic field. After the execution of the method, the first ferromagnetic layer of said at least one device has a magnetization direction that is correlated to the direction of said external magnetic field and the magnetization direction of this first ferromagnetic layer is irreversible in an external magnetic field higher than about 35 kA/m. While executing the method according to the second aspect of the invention, the substrate may also be heated.
In a fourth aspect of the present invention, a method of manufacturing a sensing system of a magnetic characteristic is disclosed. The system includes a set of magnetic devices in a balancing configuration and essentially each of said devices comprising a structure of layers including at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said first structure having at least a magneto resistance effect. The method comprises the steps of depositing a first ferromagnetic layer of at least a first of the devices of said set while applying a magnetic field to pin the magnetization direction in the first ferromagnetic layer in a first direction (the first deposition step) ; and thereafter depositing a first ferromagnetic layer of another of the devices of said set while applying a magnetic field to pin the magnetization direction in this first ferromagnetic layer in a second direction different from, preferably opposite to, the magnetization direction in the first ferromagnetic layer of the first device (the second deposition step). While executing the method according to the fourth aspect of the present invention, magnetic fields of opposing direction can be applied during the first and the second deposition step. The set of magnetic devices in a balancing configuration can include two full Wheatstone bridge arrangements, the magnetization directions in corresponding devices of the Wheatstone bridges being pinned at different angles.
In a fifth aspect of the present invention, a deposition holder for a substrate for depositing a structure of layers on said substrate is disclosed, said holder containing magnetic elements for applying an external magnetic field over at least part of said substrate, said external magnetic field having at least one magnetic characteristic that is position dependent over said substrate. Said characteristic can include the magnitude and/or the magnetization direction of said magnetic field. The deposition holder can further comprise at least one heating element for heating at least part of said substrate while applying said external magnetic field over said substrate. The deposition holder can also comprise magnetic elements for applying said external magnetic field.
In a sixth aspect of the present invention, a method of manufacturing a sensing system of a magnetic characteristic is disclosed. The system includes a set of magnetic devices in a balancing configuration and essentially each of said devices comprises a structure of layers including at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said structure having at least a magneto resistance effect. The method comprises the step of depositing a first ferromagnetic layer of said set of devices, said first ferromagnetic layer being part of an AAF-structure, thereafter orienting said first ferromagnetic layer of said set of devices, and thereafter depositing the other layers of the AAF-structure and said second ferromagnetic layer and said separation layer of a non-magnetic material. The step of orienting the first ferromagnetic layer for example can be done by heating the set of devices in a spatially varying magnetic field.
In a seventh aspect of the present invention, a method of operating a magnetic system is disclosed, said system including a set of magnetic devices in a balancing configuration and essentially each of said devices comprising a structure of layers including at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said first structure having at least a magneto resistance effect, the method comprising the step of alternating at least one of the magnetic characteristics of at least one of the devices of said set by heating said at least one device of said set while applying an external magnetic field over at least part of said system, the part including said at least one device.
Preferably the system is a sensing system of a magnetic characteristic and includes a set of magnetic devices in a balancing configuration. The system may also be a read head or a data storage system such as a MRAM memory.
In an eighth aspect of the present invention, a method of resetting or repairing or changing a magnetic system is disclosed, said system including a set of magnetic devices and essentially each of said devices comprising a structure of layers including at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said first structure having at least a magneto resistance effect, the method comprising the step of alternating at least one of the magnetic characteristics of at least one of the devices of said set by heating said at least one device of said set while applying an external magnetic field over at least part of said system, the part including said at least one device. Preferably the system is a sensing system of a magnetic characteristic and includes a set of magnetic devices in a balancing configuration. The system may also be a read head or a data storage system such as a MRAM memory.
In a ninth aspect of the present invention, a data storage system is disclosed that comprises one or more magnetic devices in a cell configuration and essentially each device comprising a structure of layers including at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said first structure having at least a magneto resistance effect, and wherein the magnetization direction of the first ferromagnetic layer of at least one of said devices is irreversible in an external magnetic field higher than about 35 kA/m. The external magnetic field can also be higher than about 40 or 50 or 60 kA/m. The external magnetic field can also be in range of about 35 kA/m to about 200 MA/m.
The sensing system of the invention can be a magnetic sensor device or a magnetic read-head such as a GMR thin film head for hard disks or any such system including a magnetic device and signal processing electronics for processing the signal of the magnetic characteristic or a measure or derivate thereof. The devices of the sensing system and the data storage system of the invention can be made in a multilayer configuration building further on the basic GMR- or TMR-stack of the device. Therefore at least part of the system is manufacturable without significantly changing a standard production process to thereby make at least part of the system at a low cost. It is possible in an embodiment of the invention to integrate the whole sensing system and/or the data storage system of the invention on an Alsimag (a mixture of oxides) slider or on one semiconductor (silicon) chip with the multilayer configuration being grown or deposited on the chip. The multilayer configuration can be grown or deposited on the chip in the front-end or in the back-end of the process for making the chip. In the back-end process a part of the chip is planarized and the multilayer configuration is deposited or grown thereon. Appropriate connections by bonding or via structures are made in order to transfer the signals of the multilayer configuration to the part of the chip containing the signal processing logic. In the front-end process, the multilayer configuration is directly integrated on the semiconductor (silicon). The sensing system of the invention can also be an integrated circuit with a memory functionality and an integrated sensing system or an ASIC with an embedded non-volatile magnetic memory element and a sensing system or a chipcard with a sensing system or any such sensing system. The set of structures of the sensing system of the invention can be made in a multilayer configuration building further on the basic GMR- or TMR-stack of the system. The layers of the devices of the systems of the invention can be deposited by Molecular Beam Epitaxy or MOCVD or sputter deposition or any such technique known to the person of skill in the art.
It is also possible to apply part of, or the whole of the teaching of the invention, to a single magnetic device or a set of magnetic devices that is not in a balancing configuration. Thus the second aspect of the invention for example can be described as a method of manufacturing magnetic device, and the device comprising a structure of layers that includes at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said structure having at least a magneto resistance effect. The method comprises the step of heating said device while applying an external magnetic field over said device. The heating can be achieved by applying a current pulse on or through the device. Preferably, the device or at least one of the devices of the set is heated to a temperature in the range of 50 to 800xc2x0 C., preferably in the range of 100 to about 300 or 400 or 600xc2x0 C. while said external magnetic field can have a value in the range of about 35 kA/m or 40 kA/m or 50 kA/m to 200 MA/m.
A number of terms that is introduced hereabove and that is used in the sequel, can be described as follows, completing the understanding by the person of skill in the art of these terms.
The term that the magnetization direction of a magnetic device is irreversible in an external magnetic field means that the magnetization direction may be changed under the influence of an external field, but once the external field is switched off, the magnetization direction is taking substantially its original position from before the external field was applied. The magnetization direction may also remain substantially unchanged while exposed to the external field. Thus the possible change of the magnetization direction is reversible. This irreversible characteristic is visible while evaluating the device at room temperature and for evaluation times that are of the order of one minute or some minutes.
A set of magnetic devices that is in a balancing configuration means that the configuration is such that in the response of the system that is made out of the devices, one can clearly distinguish between the response or resistance changes due to (varying) magnetic field and the sensor response or resistance changes due to environmental factors such as temperature variations. One approach in configuring such balancing configuration consists in connecting a number of GMR- or TMR-devices in a Wheatstone-bridge configuration.